Cathode



May 17, 1932.

F. S. MCCULLOUGH CATHODE Filed June 4. 1928 MM fw' ences between the Patented May 17, 1932 FREDERICK S. MCCULLOUGH, 0F EDGEWOOD, PENNSYLVANIA CATHODE ppm-tien md :une 4, 192s. serial No. 282,524.

My invention relates to thermionic apparatus, and its broad purpose is to provide a cathode having high emission relative to the power used to excite it. l

An object of my invention is to provide a cathode whose emissivity is not destroyed by heavy overloads of short duration.

Another object of my invention is to provide a cathode whose large effective surface permits a large space current with a relatively small density of space charge.

Another object of my invention is to provide a cathode utilizing the combined effect of thermionic emission and secondary emission from bombardment.

My invention possesses other objects and valuable features, some of which will be set forth in the following description of my invention which is illustrated in the drawings forming part of the specification. It is to be understood that I do not limit myself to the showing made by the said description and drawings, as I may adopt varying forms of my invention within the scope of the claims.

Referring to the drawings:

Figure 1 is an axial sectional. view of a thermionic rectifier embodying my invention.

Figure 2 is a view of a modified form of the cathode ofmy invention together with a section of the surrounding anode..

Broadly considered my invention comprises a secondary cathode having an electron emitting surface, such as a rare earth oxide or an alloy of thorium. Embracing this member and preferably supported by it is a heating element or primary cathode which is also capable of emitting electrons when raised to operating temperature, as for example tungsten. Radiation from the heating element raises the temperature of the secondary cathode to cause it to emit, and this effect is augmented by the heating action of electronic bombardment due to potential differprimary and secondary cathodes. Moreover secondary electrons dislodged by the bombardment join those due to thermionic emission in current.

For" purposes of detailed description I producing the space have chosen a thermionic rectifier as a preferred embodiment of my device, but it is to be understood that this use is merely illustrative, as cathodes embodying my invention may be used in vacuum relays, X-ray apparatus, and other types of apparatus where thermionic emission is desired. Such a rectifier comprises an evacuated envelope 6, into which is sealed a stem 7 having a press 8 which holds the cathode structure.

Sealed through the press is a pair of filament leads 9 and a rod 11 which forms thesecondary cathode. The seals are preferably made to tubular sleeves 12 which surround the leads within the press and are welded to them at one end, as is described in my copending application, Serial No. 269,439, filed April 12, 1928. The rod 11 is coated, in the portion lying within the tube, with one of the well known low temperature emitters, such as thoria, uranium oxide, or metallic thorium. These substances are also ready emitters of secondary electrons by electronic bombardment.

- Carried at the end of the rod 11 is an insulating bead 13, which may be of glass or fused quartz, and the bead in turn supports a short rod 14 towhich is welded a conducting yoke 16.

Supported between the yoke 16 and the filament leads 9, are the filaments 17 which comprise the heating element. The filaments are preferably helical, although any arrangement which so embraces therod as to give a good thermal transfer thereto may be used. The filament should itself be an electronl emitter at its operating temperature, and may be of solid tungsten or of other metal or alloy suitable for such use.

Surrounding the filament is a cylindrical anode 18, which is supported by a sleeve 19 fitting within a tube 21 sealed to the side of the envelope 6. An anode lead 22 connected to the sleeve is sealed through the end of the tube 21 similarly to the other leads.

In operation the secondary cathode may be maintained at a positive potential with respeet to the filament. Most of the emission from the filament is drawn directly4 to the anode. A portion, however, is drawn to the secondary cathode, which it heats b bombardment. Further heating takes p ace by thermal radiation, and the two heating effects combine to produce the necessary temperature to cause thermionic emission. The bombardment at the same time produces secondary electrons which combine with the thermionic emission to produce the space current.

Figure 2vshows a modified cathode structure in which the insulating bead 13 is omitted. When this structure is used the secondary cathode is necessarily at the mean potential of the filament. At the ends of the lament, however, the difference of potential with reference to the secondary cathode is suflicient to produce a bombardment and the result is much the same as with the first form. In this modiiication the secondary cathode v26 terminates in a clamp 27 which supports the filament 28. Thermal conduction through the clamp adds to the heating of the rod and increases the emission.

I claim:

1. A cathode comprising a support member especially adapted for emitting electrons having an electron emitting surface, and a suspended helical heating element surrounding and supported by said member, said heatifng element having an electron emitting surace.

2. A cathode comprising a support member having an electron emitting surface, an insulator on said support member, and a heatingr element having an electron emitting surface embracing said support member and borne by said insulator.

3. An electrical discharge device comprising a press, a support rod held by said press and having an electron emitting surface, filament leads extending through said press, a filament disposed about said rod and connected to said leads, an insulator mounted on said rod, and filament support means mounted on said insulator.

4. A thermionic tube comprising an anode, and a cathode comprised of a coated electron emitting member and a resistance heater wire constituting an emitting element suspended about the said member between said member and the anode and adapted to energize the said member by thermal radiation and bombardment, said member and heater wire being in an electrically common circuit'.

5. An electron tube comprising an anode, a cathode comprising a supporting post of conducting material adapted to emit electrons, and a resistance heater wire coiled about the post and supported thereon in spaced relation thereto and interposed between the post and the anode, both the heater and the post being adapted to emit electrons Y to the anode of the tube.

6. A thermionic tube comprising an anode,

a cathode structure extending substantially coextensively of the anode and coaxially FREDERICK S. MOCULLOUGH. 

